Conductive coil, stator, axial flux electric machine and method

Abstract

Provided are a conductive coil 12 of a yokeless axial-flux electric machine stator 1 having distributed windings and flux guides 30, a stator 1 including a plurality of such coils, a yokeless axial-flux electric machine 100 including the stator 1, and a method 500 of manufacturing the stator 1. The conductive coil 12 includes a first active portion 121a and a second active portion 121b, each of which includes a plurality of winding turn portions 131a, 131b extending in a generally radial direction substantially perpendicular to the axis of rotation of the electric machine and stacked parallel to the axis of rotation such that a cross section of each active portion 121a, 121b perpendicular to the radial direction is elongated with its long dimension parallel to the axis of rotation. The second active portion 121b is circumferentially spaced apart and axially offset from the first active portion 121a.

Description

[Technical field]

[0001] The present invention relates to axial-flux electric machines, and in particular to a stator for an axial-flux electric machine. [Background technology]

[0002] Electric machines, such as electric motors and generators, are already in very widespread use. However, reliance on fossil fuels to power internal combustion engines and concerns about the pollution they cause are creating political and commercial pressures to open the use of electric machines to new applications and to expand their use in existing applications. Electric machines are increasingly being used in vehicles such as electric cars, motorcycles, boats, and aircraft. They are also used in energy generation applications, for example as generators in wind turbines.

[0003] To meet the demands of these applications, electric machines need to be designed with both suitable performance characteristics such as speed and torque, as well as high efficiency. Electric machine efficiency is critical in almost all applications, for example, to increase the range of electric vehicles while reducing the required battery capacity. Reducing the required battery capacity can further reduce the vehicle weight, which leads to further efficiency improvements.

[0004] One known type of electric machine is the axial flux machine. As the name suggests, the direction of the magnetic flux lines cut during operation of an axial flux machine is parallel to the axis of rotation of the machine. This is in contrast to radial flux machines, where the direction of the magnetic flux lines cut during operation of the machine is perpendicular to the axis of rotation of the machine. Although radial flux machines are more common, axial flux machines have been used in some applications where their form factor (relatively small in the axial direction) and performance characteristics (such as high torque-to-weight ratio) are appreciated. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 015293 Summary of the Invention [Problem to be solved by the invention]

[0006] An example of a yokeless axial-flux machine utilizing a concentrated winding arrangement is described in U.S. Pat. No. 5,399,633. The stator assembly of the axial-flux machine includes separate circumferentially distributed stator teeth, each having a ferromagnetic body wound with an electrical winding. This is generally referred to as a yokeless segmented armature machine. The radially inwardly extending elongated portion of the stator housing is for cooling and provides a structure to house the stator teeth. Although this type of axial-flux machine can achieve high efficiency, it would be desirable to improve the efficiency, especially over a wider range of operating parameters. Furthermore, although the radially inwardly extending elongated portion of the housing provides some structure to house the separate stator teeth, there are difficulties associated with accurately positioning and joining each stator tooth within the stator housing, and each stator tooth must be wound on a bobbin-like structure that includes the ferromagnetic body. It would be desirable to provide a stator that can be assembled more easily and accurately. [Means for solving the problem]

[0007] The embodiments described herein provide a conductive coil and stator for an axial flux machine with multiple conductive coils that provides high mechanical efficiency, ease of manufacture, and good thermal conduction from the coils to the stator housing to aid in cooling.

[0008] In this disclosure, unless otherwise specified, terms such as "radial," "axial," "circumferential," and "angular" are used with respect to a cylindrical polar coordinate system (r, θ, z) in which the direction of the axis of rotation of the electric machine is parallel to the z-axis. That is, "axial" means parallel to the axis of rotation (i.e., along the z-axis), "radial" means any direction perpendicular to the axis of rotation, "angle" is the azimuthal angle θ, and "circumferential" refers to the azimuthal direction about the axis of rotation.

[0009] Terms such as "radially extending" and "axially extending" should not be understood to imply that a feature must be exactly radial or exactly axially parallel. For example, it is known that the Lorentz force experienced by a current carrying conductor in a magnetic field is greatest when the direction of the current is exactly perpendicular to the direction of the magnetic flux, but current carrying conductors also experience the Lorentz force at angles less than 90°. Thus, deviations from parallel and perpendicular orientations do not change the underlying principles of operation.

[0010] The invention is defined in the independent claims referred to below. Preferred features are set out in the dependent claims.

[0011] According to a first aspect of the invention, there is provided a conductive coil for a yokeless axial-flux electric machine stator having distributed windings. The conductive coil comprises a first active part and a second active part. Each active part includes a plurality of winding turn portions extending in a generally radial direction substantially perpendicular to an axis of rotation of the electric machine and stacked parallel to the axis of rotation such that a cross section perpendicular to the radial direction of each active part is elongated with a major dimension parallel to the axis of rotation. The second active part is circumferentially spaced and axially offset from the first active part.

[0012] This type of conductive coil allows for easy manufacturing of stators constructed using the conductive coils while providing high mechanical efficiency. For example, the conductive coils can form structures on which flux guides, such as lamination packs, can be placed. This allows for quick and precise manufacturing of the stator, improving the efficiency of the electric machine. Additionally, the axial offset of the active section facilitates stacking of the coils in the axial and circumferential directions. The use of axially stacked winding turns also reduces skin and proximity effects in the active section, since the cross section of each winding turn is reduced and, assuming the winding turns are connected in series, the current is deterministically controlled to flow through the entire axial length of each active section. This reduces heating and improves flux linkage.

[0013] According to this first aspect, each conductive coil may optionally comprise a plurality of pairs of active parts connected in series with each other. Adjacent pairs of active parts may overlap in the circumferential direction to define a second type of space housing the flux guide. The second type of space is a circumferential space between two adjacent active parts of different active part pairs of the coil. The circumferential space may extend substantially radially and be radially elongated like the active parts that define it. Each additional such pair of active parts per coil advantageously increases the number of slots per pole per phase by one. This can reduce losses and therefore improve efficiency, since a more precisely sinusoidal flux density can be obtained with a larger number of slots per pole per phase. Furthermore, the number of active parts per coil can be varied according to the radius of the machine.

[0014] According to a second aspect of the invention, there is provided a conductive coil of a stator of a yokeless axial-flux electric machine. The conductive coil comprises two pairs of active parts. Each active part extends in a generally radial direction substantially perpendicular to a rotation axis of the electric machine. The generally radially extending active parts of each pair are spaced apart in a circumferential direction. The two pairs of active parts partially overlap in a circumferential direction to define a second type of space for housing a flux guide. The second type of space is a circumferential space between two adjacent active parts of different active part pairs of the coil. The circumferential space is substantially radially extending and may be elongated like the active parts that define it.

[0015] Conductive coils according to the second type allow easy manufacture of stators constructed with them and provide high mechanical efficiency. For example, if several such coils are distributed circumferentially around a stator ring, the resulting coil structure has circumferentially distributed spaces (of the second type) in which flux guides can be provided. This allows stators to be manufactured quickly, with a large number of flux guides, and with high precision, thus increasing the efficiency of the electric machine. Furthermore, since each coil has (at least) two pairs of spaced active parts, the coils provide the stator with (at least) two slots per pole per phase, which makes the magnetic flux density generated by the stator more sinusoidal and with less harmonic content. With sinusoidally varying currents, the average torque generated by the electric machine is obtained from the interaction of the fundamental magnetic field components and not from the harmonic components. This is advantageous because the harmonic components of the circumferential spatial magnetic flux density lead to high eddy currents in the rotor's permanent magnets, resulting in high losses and increased heating. Furthermore, the addition of harmonic components to the winding magnetomotive force distribution can lead to increased losses in the flux guides. Furthermore, the number of active pairs per coil can be varied according to the radius of the machine and / or by selecting the span (pitch) between the actives forming each pair. Thus, each additional such active pair per coil adds another slot per phase per pole, resulting in higher efficiency, especially as machine size increases.

[0016] According to this second aspect, each active part may optionally include a plurality of winding turn portions stacked parallel to the axis of rotation such that a cross section perpendicular to a radial direction of each active part is elongated with a major dimension parallel to the axis of rotation. The axial stacking of the insulated winding turns reduces skin and proximity effects in the active part, which reduces heating and improves flux linkage as the current spreads more evenly over the conductive cross section.

[0017] According to the second aspect, each pair of active parts can be axially offset from each other as desired. Axial offsetting of the active parts facilitates stacking of the coils in the axial and circumferential directions, providing freedom of span (pitch) between each pair of active parts, and improving the structural rigidity of the complete winding due to coil intermeshing. This also increases the flux linkage of the core.

[0018] The following optional features may also apply to the conductive coil of the first embodiment and the conductive coil of the second embodiment.

[0019] In use, current flows in opposite radial directions along the active parts forming an active part pair (i.e. current flows along the second active part in the opposite direction to the current flowing along the first active part).

[0020] Each active part may be only one winding turn wide. Alternatively, each active part may be multiple winding turns wide. That is, each active part may include multiple circumferentially stacked winding turn portions. When each active part includes multiple circumferentially stacked winding turn portions, it is preferred that the number of circumferentially stacked winding turn portions is less than the number of axially stacked winding turn portions, and the major dimension of the cross section of the coil perpendicular to the radial direction of the active part is parallel to the axis of rotation. For example, the active part may be only two winding turn portions wide, but may include three or more winding turn portions in the axial direction. For example, the ratio of the number of axially stacked winding turn portions to the number of circumferentially stacked winding turn portions may be 3 or more, preferably 5 or more, more preferably 7 or more. A coil that is larger than one winding turn wide increases the overall length of the conductor, which further increases the impedance of the coil. A higher impedance may allow the use of a controller with a lower switching speed, which may potentially reduce costs.

[0021] The winding turn portions of the first and second generally radially extending active portions of the pair of active portions may have proximal ends positioned at an inner radius and distal ends positioned at an outer radius, the proximal ends of the winding turn portions may be connected by an inner loop portion and the distal ends connected by an outer loop portion such that, in use, current flows in radially opposite directions along the pair of radially extending active portions.

[0022] The outer loop portion may be configured to define an outer portion of the coil that is substantially parallel to the axis of rotation. The ability to axially insert the parallel portion of the coil into an opening in the stator housing improves ease of stator manufacture. Additionally, the flared nature of the outer portion of the coil provides mechanical locking of the coil and a larger surface area for cooling at the periphery of the stator.

[0023] Each outer loop portion may have any shape, but is preferably substantially semicircular or rectangular, such that the outer portion of the coil is a semidisk-shaped or rectangular surface. The surface of the outer portion may also be curved, for example an involute shape. These surfaces provide a large surface area, but also require a relatively limited conductor length, thereby reducing material costs.

[0024] Additionally or alternatively, the outer loop portion may be configured to form a substantially involute portion of the coil, which maintains a substantially constant gap between adjacent conductive elements, resulting in a radially interlocking arrangement of circumferentially distributed coils. There may be two substantially involute outer portions of the coil connecting the outer portions of the coil to the two active portions.

[0025] The inner loop portion may be configured to form an inner portion of the coil that is substantially parallel to the axis of rotation. Because it is substantially parallel to the axis of rotation, the inner portion occupies a minimal amount of circumferential space. This is important because physical space is limited at the inner radius of the stator.

[0026] Each inner loop portion may have any shape, but is preferably substantially semicircular or rectangular, such that the inner portion of the coil has a semidisk-like or rectangular surface. The surface of the inner portion may also be curved, for example an involute shape. These shapes reduce material costs by requiring a relatively limited conductor length to implement.

[0027] The inner loop portion may be configured to form a substantially involute portion that maintains a substantially constant gap between adjacent conductive elements resulting in a radially interlocking arrangement of circumferentially distributed coils. There may be two substantially involute inner portions of the coil connecting the inner portion of the coil to the two active portions.

[0028] The number of active pairs can be an integer multiple of 2. Using an integer multiple of 2 makes it easier to fabricate each coil from multiple identical conductive elements, reducing manufacturing costs.

[0029] The conductive coils may be configured such that, in use, current flows in the same direction along adjacent active parts of the coils that are separated by one of the flux guiding spaces, thereby avoiding current flow through those adjacent active parts having an adverse effect on torque generation.

[0030] The pairs of active parts that make up one coil can be integrally formed or can be formed by connecting in series several separate elements, each containing a pair of active parts. The connection can be made, for example, with ferrules, by brazing, or by welding. The separate elements can be formed by winding, joining, and forming conductors, which can be done using relatively inexpensively implemented techniques. Although integrally forming the elements can be expensive, it can also allow more complex coil topologies that are not or are difficult to achieve with normal winding techniques. Furthermore, integrally formed elements reduce the number of components of the stator.

[0031] The conductive coil may comprise first and second connection parts for connecting the conductive coil to a power supply. The first and second connection parts may extend parallel to the axis of rotation. The connection parts may extend in the same parallel direction or in opposite parallel directions. Parallel extending connection parts allow a very simple connection of the coil to a power supply.

[0032] The first and second connection portions of the coil may be provided proximate the axially outer ends of the coil. Thus, for example, if there is a circumferential space greater than the inner radius of the stator assembly, the connections can be made proximate the outer radius of the stator assembly. This means that the connections are less crowded, making manufacturing easier and providing a more reliable electrical connection.

[0033] There is also provided a stator for an axial-flux electric machine comprising a plurality of conductive coils according to the first aspect. There is also provided a stator for an axial-flux electric machine comprising a plurality of conductive coils according to the second aspect. In either case, the plurality of conductive coils are distributed circumferentially around the stator.

[0034] The plurality of conductive coils may be arranged in a number of groups, each group corresponding to one pole of the stator.

[0035] Each conductive coil may be configured to be connected to a phase of a multi-phase power source.

[0036] Circumferentially adjacent conductive coils may be configured to be connected to different phases of a multi-phase power supply such that for an N-phase power supply, the stator includes multiple groups of N conductive coils, each group of N conductive coils including one coil for each phase of the N-phase power supply, with each group corresponding to one pole of the stator.

[0037] For each phase of a polyphase power supply, every other coil of the stator connected to that phase may be connected to a common busbar. In this manner, the windings may be divided into two interleaved sections with half of the total number of coils per phase connected to one of the two-phase busbars.

[0038] Circumferentially adjacent conductive coils may overlap circumferentially to define a first type of space housing the flux guide. Each first type of space may be a circumferential space between two adjacent active parts of two different coils. Like the active parts that define them, the first type of spaces may extend radially and be radially elongated. Since the coils of the stator naturally form a structure housing the flux guide, the stator can be manufactured quickly and with high precision, improving the efficiency of the electric machine.

[0039] The stator may further comprise flux guides positioned in the first and / or second type spaces.

[0040] The stator may further comprise a stator housing that may include circumferentially distributed, axially extending openings for receiving outer portions of the conductive coils substantially parallel to the axis of rotation, as described above, allowing easier and more accurate manufacture and heat transfer from the conductive components of the stator through the stator housing.

[0041] An axial-flux electric machine comprising such a stator is also provided. The axial-flux machine may further comprise a pair of opposing rotors disposed on either side of the stator. Each rotor may be dedicated to one stator, or one or more rotors may be shared between the two axially aligned stators.

[0042] A method of manufacturing a stator for an axial-flux electric machine is also provided. The method includes positioning a plurality of conductive coils in a stator housing so that the conductive coils are distributed circumferentially around the stator housing. Each conductive coil includes a first active portion and a second active portion, each active portion including a plurality of winding turn portions extending in a generally radial direction substantially perpendicular to an axis of rotation of the electric machine and stacked parallel to the axis of rotation such that a cross section perpendicular to the radial direction of each active portion is elongated with a major dimension parallel to the axis of rotation. The second active portion is circumferentially spaced and axially offset from the first active portion.

[0043] The stator housing may include a plurality of circumferentially distributed, axially extending openings, in which case positioning the plurality of conductive coils within the stator housing may include, for each conductive coil, positioning an axially extending portion of each coil in one of the axially extending openings, thereby improving ease of assembly, accuracy of assembly, mechanical locking, and cooling and efficiency in use.

[0044] Each conductive coil may include multiple pairs of active parts connected in series with each other, adjacent pairs of active parts overlapping in the circumferential direction to define a second type of space housing the flux guide. The second type of space is a circumferential space between two adjacent active parts of the same coil but different active part pairs of that coil. The method further includes positioning the flux guide in the space. As mentioned above, each additional pair of active parts per coil advantageously increases the number of slots per pole per phase by one, which may reduce losses and therefore improve efficiency. Furthermore, the number of active parts per coil may be increased with the radius of the machine, allowing for improved efficiency in larger machines.

[0045] Another method of manufacturing a stator for an axial-flux electric machine is provided. The method includes positioning a plurality of conductive coils in a stator housing such that they are circumferentially distributed around the stator housing. Each conductive coil includes two pairs of active parts, each active part extending in a generally radial direction substantially perpendicular to a rotational axis of the electric machine. The generally radially extending active parts of each pair are circumferentially spaced apart. The two pairs of active parts partially overlap in a circumferential direction to define a second type of space that houses a flux guide. The second type of space is a circumferential space between two adjacent active parts of different active part pairs of the same coil. The method further includes positioning a flux guide in the space.

[0046] In this second method, each active section may include multiple winding turns stacked parallel to the axis of rotation such that a cross section perpendicular to the radial direction of each active section is elongated with its major dimension parallel to the axis of rotation. This reduces heating as the current spreads more evenly across the conductive cross section. The active sections of each pair may be axially offset from one another. Axial offsetting of the active sections facilitates axial and circumferential coil stacking, allowing span (pitch) freedom between each pair of active sections, and also improves the structural rigidity of the complete winding due to coil intermeshing.

[0047] In both methods, the conductive coils are positioned such that circumferentially adjacent conductive coils overlap circumferentially to define a first type of space that houses the flux guide. Each first type of space may be a space between two adjacent active portions of two different coils. Both methods may further include positioning a flux guide in the first type of space.

[0048] Both methods may further comprise the step of impregnating at least a portion of the stator with a bonding compound, such as a resin, which strengthens the stator assembly and protects it from mechanical and electromagnetic forces experienced during use. Advantageously, the means for connecting the coil to a power source may not be impregnated with bonding compound, leaving the connections accessible after impregnation.

[0049] Any feature of one aspect of the invention may be applied to any other aspect of the invention in any suitable combination. In particular, a method aspect may be applied to an apparatus aspect and vice versa. Furthermore, any, some, and / or all of the features of one aspect may be applied to any, some, and / or all of the features of any other aspect in any suitable combination.

[0050] It will also be understood that specific combinations of the various features described and defined in any embodiment of the present invention can be implemented and / or provided and / or used independently.

[0051] Embodiments of the invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0052] [Figure 1A] FIG. 1 is a side view of an axial flux machine showing the stator assembly, rotor, and shaft. [Figure 1B] FIG. 1B is a perspective view of the axial flux machine of FIG. 1A. [Figure 2A] FIG. 2 is a perspective view of the rotor and shaft of the axial flux machine of FIGS. 1A and 1B. [Figure 2B] FIG. 2B is a plan view of one rotor of the axial-flux machines of FIGS. 1A, 1B, and 2A, showing more clearly the permanent magnets of the rotor. [Diagram 3] FIG. 2B is a cross-sectional side view of an axial flux machine showing further details not visible in FIGS. 1A, 1B and 2A, 2B. [Figure 4A] FIG. 1 is a perspective view of the conductive components of a stator assembly of an axial flux machine that includes 48 conductive coils. [Figure 4B] FIG. 4B is a side view of the conductive components of the stator assembly of FIG. 4A. [Figure 4C] FIG. 4C is a plan view of the conductive components of the stator assembly of FIGS. 4A and 4B. [Figure 5A] 1A and 1B show top and bottom views of a single conductive coil element having a pair of radially extending active portions. [Figure 5B] 5B shows two perspective views of the conductive coil element of FIG. 5A. [Figure 5C] 5A and 5B show two side views of the conductive coil element of FIG. 5B. [Figure 5D] 5A-5C depict front and rear views of the conductive coil element. [Figure 5E] FIG. 5B is a plan view of a portion of a stator including a plurality of the conductive elements of FIGS. 5A-5D distributed circumferentially around the stator, illustrating the spacing resulting from the overlap. [Figure 5F] FIG. 5F is a plan view showing the stator of FIG. 5E. [Figure 5G] 1 is a plan view of a conductive element illustrating how the conductive element may be wound onto a flat surface. [Figure 5H] FIG. 5C is a side view of the conductive element shown in FIG. 5G. [Figure 5I] FIG. 5C is a perspective view of the conductive element shown in FIGS. 5G and 5H. [Figure 5J] FIG. 13 is a plan view of an alternative conductive coil element. [Figure 5K] FIG. 5J is a plan view of a stator utilizing the multiple conductive coil elements of FIG. 5J. [Figure 6A]1A and 1B show top and bottom views of a conductive coil including two pairs of series-connected circumferentially overlapping, radially extending active portions. [Figure 6B] 6B shows two perspective views of the conductive coil of FIG. 6A. [Figure 6C] 6A and 6B show two side views of the conductive coil pair of FIG. 6A and FIG. 6B. [Figure 6D] 6A-6C depict front and rear views of the conductive coil. [Figure 7A] FIG. 7 is a front view showing the conductive coil of FIGS. 6-6D connected to a pair of bus bars. [Figure 7B] FIG. 7 is a perspective view of the conductive coil of FIGS. 6A-6D connected to a busbar pair. [Figure 7C] FIG. 7 is a plan view of the pair of conductive coils of FIGS. 6A-6D connected to a busbar pair. [Figure 8A] FIG. 1 is a perspective view of eight conductive coils connected to the same busbar pair. [Figure 8B] FIG. 1 is a plan view of eight conductive coils connected to the same busbar pair. [Figure 9A] FIG. 13 is a front view of two circumferentially adjacent conductive coils connected to each busbar pair. [Figure 9B] FIG. 13 is a perspective view of two circumferentially adjacent conductive coils connected to each busbar pair. [Figure 9C] FIG. 13 is a plan view of two circumferentially adjacent conductive coils connected to each busbar pair. [Figure 10] FIG. 11 is a perspective view of six adjacent conductive coils illustrating an alternative method of connecting the conductive coils to a three-phase power source. [Figure 11A] FIG. 2 is a plan view of one half of a conductive component of a 16-pole, three-phase stator assembly including 24 conductive coils, each having two pairs of axially extending conductive portions. [Figure 11B] FIG. 11B is a perspective view of the stator assembly of FIG. [Figure 12A] FIG. 2 is a perspective view of a stator assembly including a stator housing that houses the conductive coils of the stator assembly. [Figure 12B]FIG. 12B is a top view of the stator assembly of FIG. 12A showing how the conductive coils are housed within the stator housing openings. [Figure 12C] FIG. 12C is a perspective view of the stator assembly of FIGS. 12A and 12B showing the bus bars and phase connections. [Figure 13] 4 is a flowchart illustrating a method of manufacturing a stator. [Figure 14] 12A-12C for a range of torque and speed values. 12B-12C for a range of torque and speed values. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] Like reference numbers are used throughout the description and figures to refer to like elements.

[0054] An embodiment of the present invention will now be described with reference to an axial-flux motor 100. Although motor 100 is described, it should be understood that the present invention may be practiced with other types of axial-flux electric machines, such as generators, as well.

[0055] Axial Flux Machine Overview Figures 1A and 1B show the main components of an axial flux motor 100. The axial flux motor 100 includes a stator assembly 1, two rotors 2a, 2b arranged on either side of the stator assembly 1, and a shaft 3. The shaft includes a drive end 3a and a non-drive end 3b. The rotors 2a, 2b are fixedly mounted to the shaft 3. In use, the stator 1 of the axial flux motor 100 remains stationary and the rotors 2a, 2b and the shaft 3 rotate together relative to the stator 1. It will be appreciated that various components typically present in the motor 100, such as rotor cover plates and means for connecting the stators to a power source, have been omitted from Figures 1A and 1B for clarity.

[0056] 1A-1B show two rotors 2a, 2b and a single stator 1, it will be appreciated that other configurations are possible. For example, one of the rotors 2a, 2b may be shared between two axially aligned stators. That is, there may be two stators and three rotors, with one of the three rotors being shared between the two stators.

[0057] 2A and 2B show the rotors 2a, 2b and shaft 3 of the motor 100 without the stator assembly 1. As is particularly clear from FIG. 2B, each rotor 2a, 2b includes a plurality of circumferentially distributed permanent magnets 21, 22, 23, 24. The magnets 21, 22, 23, 24 are, for example, rare earth magnets, such as NdFeB magnets. Circumferentially adjacent magnets, such as the permanent magnets 21 and 22, have opposite polarities. That is, each north pole 23 is circumferentially adjacent to two south poles 22, 24, and each south pole 22 is circumferentially adjacent to two north poles 21, 23.

[0058] Although not visible in Figures 2A and 2B, rotors 2a, 2b are mounted such that opposing permanent magnets have opposite polarity, i.e., the north pole of rotor 2a faces the south pole of rotor 2B and vice versa. As a result, the magnets of the two rotors 2a, 2b generate a magnetic field with axial magnetic flux lines between the two rotors 2a, 2b.

[0059] As will be appreciated by those skilled in the art, the stator assembly 1 described herein is yokeless but not ironless. A yoke is an additional structural element present in some stators to guide the magnetic flux lines between the opposing poles of the rotor magnetic field. That is, the yoke completes the magnetic circuit within the stator. The axial flux machine 100 described herein utilizes a pair of opposing rotors 2a, 2b with opposing permanent magnets having opposite polarities, and therefore does not require a yoke to complete the magnetic circuit since the magnetic flux is unidirectional. Having a yokeless stator reduces the overall weight of the axial flux machine, which is highly beneficial in many practical applications. Additionally, efficiency is improved since there are no losses due to changes in magnetic flux density in the yoke area.

[0060] The circumferential (angular) separation α of the centers of two adjacent permanent magnets 21, 22 of the rotors 2a, 2b defines the pole pitch of the axial flux motor 100. Note that the average span β of the permanent magnets can be less than or equal to the pole pitch α of the motor 100. In Figs. 2A, 2B, adjacent magnets are separated by non-magnetic spacers, so that the average span β of the permanent magnets 21-24 is less than the pole pitch α of the motor 100. In one example, β is about 3 / 4 of α. The ratio of β to α can be selected to reduce the circumferential spatial harmonic distortion of the permanent magnet flux density of the stator 1. As will be appreciated, it is not necessary to provide a non-magnetic spacer so that the span β of the permanent magnets 21-24 can be less than the pole pitch α of the motor 100. For example, the permanent magnets 21-24 can be attached to the rotor with an adhesive or the like at their required spacing.

[0061] The rotors 2a, 2b shown in Figures 2A, 2B have 16 circumferentially distributed permanent magnets 21-24 and therefore 16 poles. However, this is merely an example and in practice there may be more or less than 16 poles, to some extent depending on the intended application. For example, the poles are usually present in pairs (so there is usually an even number of poles) and the number of poles is to some extent limited by the radius of the rotors 2a, 2b, which varies depending on the size of the motor suitable for the intended application. The rotors 2a, 2b may have, for example, 8 to 32 poles.

[0062] Reference is now made to Figure 3, which shows a cross-sectional view of the axial-flux motor 100 of Figures 1-2 in further detail. Only a general overview of the components of Figure 3 is provided, as the invention described herein is primarily concerned with the conductive components 10 of the stator assembly 1, which will be described in more detail below with reference to Figures 4-12. Those skilled in the art will be familiar with the components of an axial-flux machine, such as the axial-flux motor 100, and will also appreciate that not all of the features shown in Figure 3 are essential to an axial-flux machine, and that the features present can be implemented in a variety of different ways.

[0063] In addition to the stator 1, drive end rotor 2a, non-drive end rotor 2b, and shaft 3, Figure 3 shows drive end and non-drive end rotor cover plates 4a, 4b which surround the rotors 2a, 2b and generally seal the motor 100 against outside materials. A rotor spacer ring 4c separates the rotors 2a, 2b. O-ring seals 8a, 8b and dynamic seal 9 further seal the interior of the motor 100. Rotation of the rotors 2a, 2b is assisted by drive end and non-drive end bearings 6a, 6b, which maintain an air gap 5 between the permanent magnets of the rotors 2a, 2b and the stator 1. Also shown is an encoder assembly 7, which includes an encoder mount 71, an on-axis position encoder 72, and an associated encoder sensor magnet 73.

[0064] Conductive Coil and Stator 4-12, a conductive component 10 including conductive coils 12 of a stator assembly 1 will now be described. Although specific examples of particular numbers of stator poles 11, conductive coils 12, and current phases are described, it should be understood that this is not intended to limit the scope of the claims.

[0065] 12A-12C, a stator assembly 1 is shown which includes an annular or ring-shaped stator housing 20 which houses the conductive components 10 of the stator 1. The core of the stator assembly 1, where axial magnetic flux provided by the rotor magnets interacts with current flowing radially through the conductive components 10 to generate torque which rotates the rotors 2a, 2b, includes radially extending active portions of the stator conductive components 10 and flux guides 30 in the form of lamination packs. The flux guides 30, in the form of lamination packs which may include grain oriented electrical steel sheets surrounded by electrical insulation, are positioned in the spaces between the radially extending active portions of the core conductive components 10. The flux guides 30, in the form of lamination packs, serve to pass the magnetic flux generated by the permanent magnets 21-24 between the current carrying conductors.

[0066] 4A-4C, a conductive component 10 (hereafter simply referred to as "stator 10") is shown without a stator housing 20 or a flux guide 30 in the form of a lamination pack. As can be best seen from the top view of FIG. 4C, the stator 10 has a distributed winding and comprises a number (in this case 16) circumferentially distributed stator poles 11a, 11b, ..., 11p, each of which includes a number of conductive coils 12. Each conductive coil 12 is connected to one phase of a polyphase power supply via connection means 15, 16, which in this example take the form of a busbar. In this particular example, the stator 10 is configured for use with a three-phase power supply, so that there are three conductive coils 12 per stator pole 11a-11p.

[0067] With 16 poles 11a-11p and three conductive coils 12 per pole, it will be appreciated that the stator 10 of Figures 4A-4C has a total of 48 circumferentially distributed conductive coils 12. However, from the top view of Figure 4C, it can be seen that this stator 10 actually has 96 radially extending active portions. Furthermore, from the side view of Figure 4B, it can be seen that there are two axially offset layers of radially extending active portions, resulting in a total of 192 radially extending active portions. The reason for this will become clear from the explanation of Figures 5-9. In summary, each conductive coil 12 includes one or more conductive elements 120, each of which includes a pair of axially offset radially extending active portions. Each conductive coil 12 of the stator 10 of Figures 4A and 4B includes two such conductive elements 120, with each conductive element 120 including a pair of axially offset radially extending portions, resulting in a total of 192 radially extending active portions.

[0068] The conductive components of the stator 10 may be made of any combination of one or more conductive materials, however, it is preferred that the conductive components 10 are made of copper.

[0069] Figures 5A-5D show various views of a single conductive element 120. As noted above and described in more detail below, each conductive coil 12 is composed of one or more conductive elements 120. It will be understood that in the case of one conductive element 120 per conductive coil 12, the conductive coil 12 and the conductive element 120 are equivalent. Figures 6A-6D show a conductive coil 12 composed of two conductive elements 120 and 120', as described below.

[0070] 5A-5D, as best seen in the top view of FIG. 5A where the axis of rotation is perpendicular to the page, conductive element 120 includes a pair of circumferentially spaced apart, radially extending active conductive portions 121a, 121b. These radially extending active portions 121a, 121b are referred to as "active" portions because they are located within the stator core and interact with the magnetic field provided by the magnets of rotors 2a, 2b when conductive coil 12 is positioned in the stator. It will be appreciated that because the active portions extend in a generally radial direction that is generally perpendicular to the magnetic flux in the core, the flux linkage is at least close to a maximum.

[0071] The angle γ that separates the two active parts 121a, 121b is called the coil span. The coil span may be the same as the pole pitch α (defined as the angle between the centers of the permanent magnets of the rotor) or may be different (smaller or larger). Preferably, the coil span γ is less than the pole pitch α. For example, γ may be about 5 / 6 of α. By making γ less than α, a fractional pitch winding of the windings can be implemented, which reduces the space harmonic content of the winding magnetomotive force (mmf).

[0072] 5E and 5F, which show a 16-pole, 3-phase stator 10', which is similar to stator 10 of FIGS. 4A-4C, except that each coil 12 of stator 10' has only one conductive element 120 (a pair of active portions 121a, 121b). That is, in FIGS. 5E and 5F, the coils 12 and conductive elements 120 are identical. Like stator 10, the conductive coils 120a, 120b, 120c of stator 10' are distributed circumferentially around the stator, with circumferentially adjacent coils overlapping circumferentially.

[0073] As is particularly clear from FIG. 5E, the circumferential overlap of the coils 120a, 120b, 120c defines circumferential spaces between the active parts of the coils. These radially elongated circumferential spaces can accommodate the flux guides 30. The spaces labeled 141a, 141b, 141c, etc., are referred to as spaces of a first type. Thus, the spaces of the first type 141a, 141b, 141c are defined between the active parts of the different coils. For example, the space 141b is between one of the two active parts of the coil 120a and one of the two active parts of the coil 120c. However, it should be understood that the two coils that define a particular space 141a, 141b, 141c of the first type may vary depending on various factors, including the number of phases per stator pole, the number of poles, and the selected coil span γ.

[0074] 5A-5D, as can be seen in Figures 5B and 5D, the two active portions 121a, 121b are axially offset from one another. This facilitates circumferential stacking of the conductive coils 12, and also facilitates circumferential stacking of the conductive elements 120 when there are multiple conductive elements 120 per conductive coil 12. As will be described in more detail with reference to Figure 14, this allows for more stator poles and more slots per phase per pole, both of which can provide increased efficiency. Additionally, the windings can be easily made short-pitch wound.

[0075] As can be seen in each of Figures 5B, 5C, and 5D, each conductive element 120 is formed from a single continuous length of wound conductor. The outermost turn of this length of conductor terminates in a first connection portion 128, referred to as the outer tail 128. The outer tail 128 extends substantially parallel to the axial direction. This facilitates convenient connection of the coil 12 to a multi-phase power source, as will be described in more detail below. The innermost winding turn portion terminates in a second connection portion 129, referred to as the inner tail 129.

[0076] As can also be seen in each of Figures 5B, 5C, and 5D, the length of conductor forming the conductive element 120 is wound such that there are a number of winding turn portions 131a, 131b stacked parallel to the axis of rotation of the electric machine. A cross section of the conductive element 120 taken perpendicular to the radial direction of each active portion 121a, 121b is elongated with the long dimension parallel to the axis of rotation. In the example of Figures 5A-5D, there are 14 axially stacked winding turn portions 131a, 131b, although this is not intended to limit the invention as other numbers are possible.

[0077] Figures 5G, 5H, and 5I show how the conductive element 120 may be formed by winding a single conductor. As shown in Figure 5G, the conductor is wound in a single plane around a pair of support elements 301, 302 (projecting perpendicularly out of the page) to form a flat, planar winding of multiple (in this case 14) turns or layers. The flatness of the winding is best seen in Figures 5H and 5I. The innermost turn terminates in an inner tail 129 and the outermost turn terminates in an outer tail 128.

[0078] Once the flat windings shown in Figures 5G-5I are formed, the three-dimensional shape of the conductive element 120 is formed by bending or deforming the flat windings into the shapes shown in Figures 5A-5D. The bending can be performed using a bending tool as known in the art. For example, a bending tool having an axially offset inner male profile block can be pressed against an outer female block to bend the flat windings such that the active portions are axially offset from one another. The outer tails 128 and inner tails 129 can be bent separately as desired.

[0079] To facilitate the bending process, the flat windings may be initially reinforced so that the windings maintain their shape during bending. In one example, the conductor has a heat or solvent activated outer bonding layer to bond the turns / layers together after winding and allow them to maintain their shape.

[0080] 5G-5I, it should be understood that conductive element 120 can be wound in a variety of different ways and that the invention is not intended to be limited to the particular windings shown. - The windings in FIG. 5G are wound counterclockwise around the support elements 301, 302, however it is also possible to wind a single conductor in a clockwise direction. - The outermost turn of the winding is terminated, but not necessarily, such that the outer tail 128 leads to the active part 121a, 121b of the conductive element 120. The outer turn may terminate at any point on the turn, for example the outer tail 128 may lead to a loop part of the turn rather than to an active part. Although 14 axially stacked winding turns are shown in FIG. 5, there may be more or less than 14 turns. - the thickness of the winding is 1 turn / layer (see in particular FIG. 5H), but may be 2 turns / layer or more thick. In this case, each conductive element 120 will include a number of circumferentially stacked winding turn portions. Any number of circumferentially stacked winding turn portions is possible, but preferably less than the number of axially stacked winding turn portions, so that the cross section of the conductive element 120 perpendicular to the radial direction of each active part 121a, 121b still has a major dimension parallel to the axis of rotation. For example, the ratio of the number of axially stacked turns to the number of circumferentially stacked turns can be 4 or more, preferably 6 or more.

[0081] As can be seen above, in use, current flows in opposite directions (i.e. inwards and outwards parallel to the radial direction) along the two active parts 121a, 121b of the conductive element 120. Reversal of the current direction is obtained by the outer loop parts 122 of the winding turn portions 131a, 131b and the inner loop parts 125 of the winding turn portions 131a, 131b. Each of the outer loop parts 122 includes a first portion 123 and a pair of second portions 124a, 124b (one for each of the pair of active parts 121a, 121b) connecting the active parts 121a, 121b to the first portion 123. Similarly, each of the inner loop portions 125 includes a first portion 126 and a pair of second portions 127a, 127b (one for each of the pair of active portions 121a, 121b) connecting the active portions 121a, 121b to the first portion 126.

[0082] As can be seen in Figures 5B, 5C and 5D, the outer first portions 123 together form an outer portion 133 of the coil element 120 having a surface substantially parallel to the axis of rotation. In the particular example of Figures 5A-5D, the outer first portions 123 are substantially semicircular, so that the outer portion 133 is a substantially flat semicircular plate 133, although other shapes are possible. For example, each of the outer first portions 123 may have a shape corresponding to three sides of a rectangle, so that together they form the outer portion 133 having a flat rectangular surface. As another example, the outer portion 133 of the conductive element 120 formed by the outer first portions 123 need not be flat or planar, as shown in Figure 5J, which shows a conductive element 120'' having a curved profile and thus a curved outer portion 133''. FIG. 5K shows a plan view of a stator 10'' with such conductive elements, which may be equivalent to FIG. 4C (although the stator 10'' does not show the connection means 15, 16).

[0083] The surface 133 defined by the outer first portion 123 may be used to facilitate cooling through a relatively large surface area. Additionally, because the outer portions 133 of the coils 120 are substantially parallel to the axis of rotation, the stator housing 20 may be provided with axially extending openings 25 that axially receive the outer portions 133 of the coil elements 120', 120'', providing mechanical locking and improved cooling, as will be described in more detail below.

[0084] The inner first portion 126 together form the inner portion 136 of the coil element 120. The inner portion 136 shown in Figures 5B-5D is substantially similar to the outer portion 133 described above, may be parallel to the axis of rotation like the outer portion 133 described above, and may be of various shapes and profiles. However, because the inner portion 136 generally plays a lesser role in cooling and lamination of the coil 12, the inner portion 126 may be configured to reduce the total amount of conductor per conductive element 120 to reduce costs.

[0085] Regarding the outer second portions 124a, 124b and the inner second portions 127a, 127b, they appear substantially straight in Fig. 5A-5D, but are actually slightly curved. In particular, the shape of each of the outer first portions 124a, 124b is a part of a first involute, so that the first portions 124a, 124b together form the outer substantially involute portions 134a, 134b of the coil element 120. Similarly, the shape of each of the inner second portions 127a, 127b is a part of a second involute, so that the first portions 127a, 127b together form the inner substantially involute portions 137a, 137b of the coil element 120. The significance of the involute will be explained with reference to Figs. 6A-6D.

[0086] Although the conductive element 120 has been described above as being formed by winding a length of conductor, this is not required and the conductive element 120 can be manufactured in other ways, including being integrally formed.

[0087] Additionally, while the illustrated elements 120 are wound from a single conductor and include stacks of winding turn portions 131a, 131b, this is preferred but not required. For example, rather than an axially extending stack of winding turn portions 131a, 131b, each conductive element 120 can be formed from a single axially extending conductive strip. In some cases, a single axially extending conductive strip may be preferred over multiple axially stacked winding turn portions 131a, 131b, but as described below, the use of stacked winding turn portions 131a, 131b can advantageously help reduce skin and proximity effects that can lead to increased losses.

[0088] As noted above, each conductive coil 12 may include only one conductive element 120. However, for reasons described in more detail below, each conductive element preferably includes two or more circumferentially overlapping conductive elements. An example of a conductive coil including two circumferentially overlapping conductive elements 120, 120' will now be described with reference to Figures 6A-6D.

[0089] 6A shows top and bottom views of a conductive coil 12 including two conductive elements 120, 120'. The features of each of the two conductive elements 120, 120' are the same as those of the single conductive element 120 described above with reference to Figures 5A-5D, and therefore these features will not be described again.

[0090] To form the conductive coil 12, two identical conductive elements 120, 120' are electrically connected in series with each other at their inner tails 129, 129'. In the example shown here, the inner tails 129, 129' are connected using a ferrule 130. However, there are other ways to connect the inner tails 129, 129', such as brazing or welding. To connect the two elements 120, 120', one of the two conductive elements 120, 120' is rotated 180° about an axis extending perpendicular to the plane of the paper in FIG. 6A so that the outer tails 128, 128' of the two conductive elements 120, 120' are in opposite directions and the inner tails 129, 129' are adjacent and can be more easily connected to the ferrule 130. Alternatively, the conductive coil 120 including the two conductive elements can be integrally formed as a single piece.

[0091] The resulting conductive coil 12 has two pairs of circumferentially overlapping spaced apart active part pairs 121a, 121b; 121a', 121b'. In particular, the two pairs of active parts define two spaces 142a, 142b. The first space 142a is defined between one (first) active part 121a of the first conductive element 120 of the coil 12 and one (first) active part 121a' of the second conductive element 120' of the coil 12. The second space 142b is defined between the other (second) active part 121b of the first conductive element 120 of the coil 12 and the other (second) active part 121b' of the second conductive element 120' of the coil 12. That is, the two spaces 142a, 142b are circumferential spaces between adjacent active parts 121a, 121a'; 121b, 121b' of two different active part pairs 121a, 121b; 121a', 121b' of the same coil 12. This type of space is referred to as the second type of space. Like the first type of space, the second type of space 142a, 142b provides space for a flux guide 30 such as a lamination pack. This makes the stator assembly 1 easier to manufacture and also increases the number of slots per pole per phase of the stator assembly 1, thereby increasing the efficiency of the motor.

[0092] Having described a first type of space 141a-141c (i.e., spaces defined between active parts of different coils) and a second type of space 142a, 142b (i.e., spaces defined between active parts of the same coil but different pairs), it should be noted that the first and second type of spaces may coincide if multiple coils 120 are provided in the stator 10 to define the first type of space. This is seen most clearly in FIG. 11A, which shows a 16-pole, three-phase stator in which each coil 12 includes two conductive elements 120, 120′. Only half of the conductive coils 12 are shown in FIGS. 11A-B so that the spaces can be clearly seen. Whether the first and second type of spaces coincide may depend on several factors, including the selected coil span γ, the number of stator poles, and the number of phases.

[0093] 6A-6D, it can also be seen from Figures 6A and 6B that there is a gap 143a between the second portions 124a, 124a' of the outer loop portions 122, 122' that form the outer involute portions 134a, 134a' of one pair of the two conductive elements 120, 120'. Similarly, there is a gap 143b between the second portions 124b, 124b' of the outer loop portions 122, 122' that form the outer involute portions 134b, 134b' of the other pair. There is also a gap 144a between the second portions 127a, 127a' of the inner loop portions 125, 125' that form the inner involute portions 137a, 137a' of one pair. Finally, there is also a gap 144b between the second portions 127b, 127b' of the inner loop portions 125, 125' that form the other pair of outer involute portions 137b, 137b'. Due to the geometric properties of the involute, the width of these gaps 143a, 143b, 144a, 144b is substantially constant along the length of the involute portion of the conductive elements 120, 120'. This advantageously reduces the motor diameter and coil losses obtainable for a given rating.

[0094] Although the conductive coil 12 has been described with two conductive elements 120, 120', it should be understood that the conductive coil 12 may have any integer number of conductive elements 120, including three or more. Increasing the number of conductive elements per conductive coil 12 further increases the number of slots per pole per phase of the stator 1 by increasing the number of spaces of the second type defined by the circumferentially adjacent active parts of the conductive elements 120. This may lead to the generation of a stator magnetic field with a more truly sinusoidal magnetic flux density with less harmonic distortion. This advantageously reduces the generation of eddy currents in the permanent magnets of the rotors 2a, 2b, further reducing heating losses and thus resulting in higher motor efficiency. However, it will be understood that the number of conductive elements 120 per conductive coil 12 is generally limited by size constraints. For example, for a given cross section of a conductor (i.e., the cross section of the wire of the wound winding) and a given radius of the stator, there is a limited number of conductors that can be fitted circumferentially into a single coil span γ.

[0095] If the coil 12 has more than two conductive elements, some additional considerations may arise. For example: When forming a coil by connecting a number of conductive elements 120 (e.g. by ferrules 130), it may be preferable to provide several types of conductive elements to make the connection of adjacent conductive elements simpler. For example, the conductive element 120 described above may be used as the two circumferentially outer conductive elements, since its outer tail 128 is connected to the power supply. However, since one or more inner conductive elements between the outer conductive elements are connected to conductive elements at both their inner and outer tails 129, a second type of conductive element may be provided to make the connection easier, having an outer tail 128 adapted similarly to the inner tail 129. Alternatively, each coil 12 may be formed as an integral unit rather than by the connection of three or more separate conductive elements. - An integer multiple of two conductive elements 120 per coil 12 may be preferred over an odd number of conductive elements 120 per coil 12. When an integer multiple of two conductive elements 120 is used, the outer tails 128 of the two circumferentially outermost elements 120 are oriented in parallel opposite directions as shown in Figures 6A-6D. This is not required, but results in simpler connection of the coils 12 using the connection means described below with reference to Figures 7-10.

[0096] Although a stator 10 has been described with a single axial layer of circumferentially distributed coils 12 (a single layer with coils 12 having axially offset active portions), it will be understood that there can be multiple axially stacked coil layers per stator. In this case, it can be advantageous for the first type spaces and / or the second type spaces of each phase to be substantially circumferentially coincident. This advantageously allows for the insertion of axially long flux guides 30 that can penetrate the axial length of multiple axially stacked layers, further improving with regard to ease and speed of assembly.

[0097] Connecting the Coil to a Multiphase Power Supply A method for connecting the multiple circumferentially distributed conductive coils 12 to a multi-phase power supply will now be described. It should be understood that in practice there are many different ways in which this can be achieved, as will occur to those skilled in the art. The invention is therefore not limited to any particular connection configuration. However, the described method for connecting the conductive coils 12, utilizing the connection means 15, 16 axially above / below the plane perpendicular to the axis of rotation and axially above / below the conductive coils, achieves a particularly neat and orderly series of connections. Furthermore, the ease of connection reduces the possibility of connection failure and allows the stator to be resin-impregnated without impregnating the connection means, which allows the connections to be inspected and fixed even after impregnation of the stator assembly.

[0098] 4B, there is a first connection means 15 located axially above the conductive coils 12 and in a plane perpendicular to the axis of rotation of the motor 100. There is also a second connection means 16 located axially below the plane perpendicular to the axis of rotation of the motor 100 and in a plane perpendicular to the axis of rotation of the motor 100 and in a plane perpendicular to the axis of rotation of the motor 100. In the case of a stator 10 configured for use with a three-phase power supply, the connection means 15 and 16 correspond to each of the three phases. However, this can be extended to a polyphase power supply having any number of phases.

[0099] 4A-4C, referred to as a parallel connection configuration, each of the connection means 15, 16 includes three phase connections and one star connection. That is, the first connection means 15 includes a first phase connection 151 for a first phase of the power supply, a second phase connection 152 for a second phase of the power supply, a third phase connection 153 for a third phase of the power supply, and a star connection 154. Similarly, the second connection means 16 includes a first phase connection 161 for a first phase of the power supply, a second phase connection 162 for a second phase of the power supply, a third phase connection 163 for a third phase of the power supply, and a star connection 164.

[0100] In the described example, the phase connections 151-153, 161-163 and star connections 154, 164 are in the form of annular busbars whose outer periphery (but which could also be the inner periphery) substantially coincides with the axially extending outer tails 128, 128' of the conductive coils. The phase connection busbars 151-153, 161-163 are themselves connected to a power supply via inputs 1510-1530, 1610-1630.

[0101] In the illustrated parallel connection configuration, each conductive coil 12 is connected to one phase of the power supply by connecting the coil 12 to one of the phase connections of one of the connection means 15, 16 (e.g., phase connection 151) and to a star connection of the other of the connection means 15, 16 (e.g., star connection 164). The connection of one conductive coil 12 to one phase connection 151 and one star connection 164 is shown in Figures 7A-7C and will now be described with reference to these.

[0102] 7A-7C show one conductive coil 12 having two conductive elements 120, 120' connected to a first phase connection 151 from a first connection means 15 and to a star connection 164 from a second connection means 16. The outer tails 128, 128' of the conductive coil 12 extend in opposite axial directions and the circumference of the busbars 151, 164 coincide with the axially extending outer tails 128, 128' such that the outer tails 128, 128' can be easily connected to the connections 151, 164.

[0103] To facilitate easier connection, the annular busbars 151, 164 are provided with circumferentially spaced apart receiving means 151a-151h, 164a-164x for receiving the axially extending outer tails 128, 128' of the coils 12. In the illustrated three-phase parallel connection configuration, each star connection 154, 164 is connected to half of the total coils 12, while each phase connection 151-153, 161-163 is connected to only one of the six coils 12. As a result, in this example, the star connection 164 has three times more equally spaced receiving means 164a-164x than the first phase connection 151.

[0104] 4A-4C, each pole 11a-11p of stator 10 consists of one conductive coil 12 for each phase (i.e., three conductive coils 12 per pole 11a-11p because the stator is configured for use with a three-phase power supply), with circumferentially adjacent conductive coils 12 connected to different phases. This is shown in Figures 11A and 11B for a 16-pole stator 10 that is connected to a three-phase power supply, but only half of its conductors are shown so that only 24 circumferentially distributed conductive coils 12 are visible.

[0105] In this regard, in the three-phase parallel connection configuration shown in Figures 4, 7-9, 11 and 12, every fifth conductive coil 12 is connected in the same manner to the connection means 15, 16. This is shown in Figures 8A and 8B. There may be eight equally spaced conductive coils 12a-12g connected to the same phase connection 151 and the same star connection 164. Although not shown in Figures 8A and 8B, it will be understood that between each of the coils, another coil 12 is connected to the same phase of the power supply, but by a complimentary set of busbars, i.e., the phase connection 161 and star connection 154.

[0106] The conductive coils 12 corresponding to the other phases of the power supply are connected in essentially the same manner as described above for phase 1. To illustrate this, Figures 9A-9C show two circumferentially adjacent conductive coils 12 connected in a parallel-connected configuration.

[0107] 9A-9C show two circumferentially adjacent conductive coils 12a, 12b. The conductive coil 12a is connected similarly to the conductive coil 12 of FIGS. 7A-7C. That is, the coil 12a is connected to the second phase connection 152 and the star connection 164. The coil 12b is circumferentially adjacent to the coil 12a and is connected to a different phase of the power supply and thus to a different pair of busbars. Specifically, without loss of generality, the circumferentially adjacent coil 12b is connected to the third phase connection 163 of the second connection means 16 and to the star connection 154 of the first connection means.

[0108] The connections of the conductive coils 12 have been described above with respect to a parallel connection configuration. However, other connection configurations are possible. To illustrate this, Figure 10 shows an alternative configuration, which will be referred to as a series connection configuration.

[0109] In the series connection configuration of Figure 10, the first connection means 15' above the conductive coil 12 differs from the connection means 15 of Figures 4, 7-9, 11 and 12 in that it does not include a star connection but only a first phase connection 151', a second phase connection 152' and a third phase connection 153'. However, the second connection means 16' is the same as the second connection means 16 of Figures 4, 7-9, 11 and 12 in that it has three phase connections 161', 162', 163' and a star connection 164'. To compensate for the lack of a star connection in the first connection means 15', the conductive coil 12 is connected in a different manner. The phase connections 151'-153' of the first connecting means 15' also accommodate twice the number of conductive coils 12 and therefore have additional storage means equivalent to the storage means of the first and second connecting means 15, 16 in a parallel connection configuration with the second connecting means 16'.

[0110] FIG. 10 illustrates a series-connected configuration for two circumferentially adjacent stator poles 11 and 11'. As in the parallel-connected configuration, each pole 11, 11' includes one conductive coil per pole, resulting in three coils per pole. Pole 11 consists of conductive coils 12a, 12b, and 12c, and pole 11' consists of conductive coils 12a', 12b', and 12c'. Also as in the parallel-connected configuration, circumferentially adjacent coils are connected to different phases. However, the coils of the same phase but adjacent poles in the parallel-connected configuration (e.g., 12a and 12a') are essentially connected independently to form separate current paths, whereas in the series-connected configuration the connections are related and part of the same current path.

[0111] Considering only the coils 12a, 12a' connected to the same phase, the coil 12a of the first pole 11 is connected by its outer tail to the phase connection 153' of the first connection means and to the phase connection 163' of the second connection means. The coil 12a' of the second adjacent pole 11' is connected to the phase connection 153' of the first connection means 15' and to the star connection 164' of the second connection means. The current path can therefore be considered to run from the phase connection 163' through the coil 12a, then along the phase connection 153' through the coil 12a' to the star connection 164'.

[0112] Different connection configurations can be used for different practical applications. For example, the series connection configuration described above theoretically provides a mechanical torque constant (measured in Nm / A) that is twice that provided by the parallel connection configuration described above. This is certainly advantageous for some, but not all, practical applications.

[0113] Although the connecting means 15, 15' have been described as being above the coil 12 and the connecting means 16, 16' have been described as being below the coil, it should be understood that both pairs 15, 16; 15', 16' may be above the coil or both pairs 15, 16; 15', 16' may be below the coil. In this case, it may be preferable to manufacture the coil 12 in which the outer tails 128, 128' extend axially in the same direction rather than axially in opposite directions.

[0114] Additionally, although the connection means 15, 16, 15', and 16' have been described as continuous annular busbars, this is only one way of implementing the connection means. For example, the connection means need not be continuous or annular, but instead may take the form of a series of two or more circumferentially distributed busbar sections. Many other types of connection means will occur to those skilled in the art.

[0115] Stator Manufacturing The features and configuration of the conductive coils 12 described above provide for a particularly efficient and effective manufacture of a stator including a plurality of circumferentially distributed coils 12. Of particular importance is the provision of a structure in which the coils 12 themselves can be provided with flux guides 30, for example in the form of a lamination pack. This makes placement of the flux guides 30 in the stator assembly 1 a relatively simple and precise exercise, particularly compared to many known manufacturing techniques which may involve winding the coils onto a bobbin-like structure which contains a lamination pack and then separately fixing (e.g. with an adhesive) the wound bobbin-like structure into the stator housing. Various other advantages are set forth.

[0116] FIG. 13 is a flow chart illustrating a method 500 for manufacturing a stator.

[0117] The method 500 includes providing 510 a plurality of conductive coils, such as the conductive coil 12 described above. Preferably, the conductive coils 12 have circumferentially overlapping pairs of circumferentially spaced apart radially extending active portions (as in the coil 12 of FIGS. 6A-6D) such that each coil 12 provides the second type of space. However, the coil 12 may have only one pair of spaced apart active portions (as in the coil of FIGS. 5A-5D). The conductive coil 12 may be formed as a single integral part, by a series connection of multiple conductive elements 120, or in any other manner.

[0118] At 520, the method 500 includes positioning a plurality of conductive coils 12 within the stator housing such that they are circumferentially distributed around the stator housing. Preferably, the conductive coils are positioned such that circumferentially adjacent conductive coils overlap circumferentially to define a first type of space for housing the flux guide. The circumferential overlap of circumferentially adjacent coils 12 can be ensured by providing a suitable number of coils 12 with a suitable coil span γ within the housing. As mentioned above, the first and second types of spaces may coincide with each other if the coils 12 have multiple pairs of active portions each defining a second type of space.

[0119] The stator housing 20 may be provided with a plurality of circumferentially spaced, axially extending openings 25 for receiving the coils 12. This allows easier and more precise positioning of the coils 12 in the stator housing. If the coils 12 are formed with an axially extending outer portion 133, the axially extending outer portion 133 may advantageously be received within the axially extending openings 25. The large surface area of ​​the axially extending outer portion 133 provides a good mechanical locking of the coils 12 to the stator housing for assembly without the need for adhesives (for example), and also provides a cooling source for the stator. The circumferentially distributed openings 25 for receiving the coils 12 can be seen most clearly in Figures 12A-12C.

[0120] Optionally, at 530, the method 500 includes a step of positioning a flux guide 30, such as a lamination pack, in the space (first and / or second type) defined by the coil 12. As described above, the overlap of adjacent coils forms spaces 141a, 141b, 141c of the first type between the active parts of the different coils. If the coils 12 each include more than one pair of radially extending active parts (as in Figures 6A-6D), a pair of spaces 142a, 142a' of the second type is also defined in each conductive coil 12. In either case, the flux guide can also be positioned in the space. Since the coils 12 themselves provide a structure with a defined space, the positioning of the lamination pack in the structure is simple, fast and precise. Combined with the provision of openings 25 in the stator housing 20 to accommodate the coils 12, this means that both components of the stator core (the active portion of the coils 12 and the flux guides 30) can be positioned quickly and very accurately compared to many known techniques. It will be appreciated that accurately positioned core components reduce losses and therefore improve mechanical efficiency.

[0121] Optionally, at 540, the method 500 includes connecting multiple coils 12 to the connecting means 15, 16 so that they can be connected to a multi-phase power source. This can be done in any desired manner, for example using bus bars in a parallel or series connected configuration as described above.

[0122] Optionally, at 550, method 500 includes impregnating at least a portion of stator assembly 1 with a bonding compound, such as a resin, to strengthen the stator structure and protect against mechanical and electromagnetic forces experienced by stator assembly 1 during use, and may improve thermal conduction between stator components if the bonding compound has a significantly higher coefficient of heat transfer than air.

[0123] If the connection means 15, 16 are provided axially above and / or below the coils 12 as described above, the impregnation of the stator can take place before or after the connection of the coils to the connection means. Moreover, it is advantageous that if the connection means 15, 16 are not themselves impregnated, the connections can be tested, modified and, if necessary, replaced after impregnation. This is highly desirable, since a poor connection in a resin-impregnated stator can render the whole stator unusable and unfixable.

[0124] Mechanical Efficiency It has been found that the axial flux machine 100 with the stator assembly 1 described herein provides high efficiency over a wide range of operating parameters as well as high peak efficiency. High peak efficiency is often quoted but is rarely achieved in practice, especially in applications where the machine needs to function over a range of operating parameters. Hence, efficiency over a wide parameter range is a more realistic and meaningful measure for many applications.

[0125] To illustrate this, Figure 14 is an efficiency map showing efficiency measurements of an axial flux machine with the stator assembly of Figures 12A-12C over a range of torque and speed values ​​commonly used in many applications. Contours of constant efficiency are included in the efficiency map. Thus, not only is there a high peak efficiency (93%), but the efficiency remains very high throughout most of the efficiency map and remains high (above 80%) even at a relatively constant speed of 500 rpm up to a torque of 30 Nm.

[0126] There may be several different reasons for the high efficiency achievable by the stator assembly 1, some of which are explained below.

[0127] First, as mentioned above, the almost self-forming structure of the conductive components of the stator 10 imparted by the geometry of the coils 12 allows for very precise placement of the components of the stator core. Precise placement of the core components means that there is better coupling of the stator and rotor magnetic fields and greater symmetry around the stator, improving torque generation.

[0128] Another significant advantage is the generation of a stator magnetic field with a more sinusoidal flux density. As will be appreciated by those skilled in the art, the greater the number of slots per pole per phase of the stator, the closer the flux density can be to a sinusoid. The coils 12 and stator 10 described above can be configured to have a greater number of slots per pole per phase by increasing the number of conductive elements 120 per conductive coil 12, which can be easily increased (e.g., if the stator radius can be increased in a particular application). The advantage of a highly sinusoidal flux density is that the flux density has a relatively low harmonic content. With less harmonic content, the coupling of the rotor and stator magnetic fields contains more of the fundamental component of the flux density and interacts less with the harmonic components. This reduces the generation of eddy currents in the rotor magnet, which further reduces losses due to heating. In contrast, many known axial flux motors utilize concentrated winding configurations that offer only a limited number of slots per pole per phase (e.g., fractional), which generate a much more trapezoidal flux density with more harmonic content.

[0129] The coil 12 can be implemented with axially extending strips, but is preferably implemented with an axially stacked winding configuration as shown in Figures 5A-5D and 6A-6D. Many motor manufacturers may consider this a disadvantage as it may be considered to reduce the packing factor of the stator core, but the inventors have found that this is compensated for by the reduction of skin and proximity effects which cause the current to flow outside the conductor cross section and primarily in the axially outer portion of the active section. The number of axial turns may be selected to balance these two considerations.

[0130] Several embodiments having various optional features have been described above. Any combination of one or more of the optional features is possible, except for any contradictory features.

Claims

1. A conductive coil (12; 120) of a yokeless axial-flux electric machine stator (1) having flux guides and distributed windings, the conductive coil (12; 120) comprising a pair of conductive elements (120, 120') connected in series with each other and overlapping in the circumferential direction, Each conductive element (120, 120') a pair of active parts (121a, 121b) including a first active part (121a) and a second active part (121b), each active part (121a, 121b) extending in a radial direction perpendicular to a rotation axis of the electric machine (100); a plurality of winding turn portions (131a, 131b) stacked parallel to the axis of rotation such that a cross section perpendicular to a radial direction of each active portion (121a, 121b) is elongated with a long dimension parallel to the axis of rotation, the second active portion (121b) being circumferentially spaced and axially offset from the first active portion (121a); The conductive coil (12) further comprises first and second connection portions (15, 16) for connecting the conductive coil (12) to a power source.

2. 2. A conductive coil according to claim 1, wherein, in use, electrical currents flow in opposite radial directions along the first and second radially extending active portions.

3. 3. The conductive coil of claim 1 or 2, wherein each active portion further comprises a plurality of circumferentially stacked winding turn portions.

4. 4. A conductive coil as claimed in claim 2 or 3, wherein the winding turn portions of the first and second radially extending active portions have proximal ends positioned at an inner radius and distal ends positioned at an outer radius, the proximal ends of the winding turn portions being connected by an inner loop portion and the distal ends being connected by an outer loop portion such that, in use, current flows in radially opposite directions along the first and second radially extending active portions of the pair of active portions.

5. 5. The conductive coil of claim 4, wherein the outer loop portion is configured to form an outer portion of the coil that is parallel to an axis of rotation.

6. 6. The conductive coil of claim 5, wherein each outer loop portion includes a semicircular or rectangular cross-section such that the outer portion forms a semidisk-shaped or rectangular surface.

7. 7. A conductive coil according to any one of claims 4 to 6, wherein the outer loop portion is configured to form an involute portion of the coil.

8. 8. A conductive coil according to any one of claims 4 to 7, wherein the inner loop portion is configured to form an inner portion of the coil that is parallel to an axis of rotation.

9. 9. The conductive coil of claim 8, wherein each inner loop portion includes a semicircular or rectangular cross-section such that the inner portion forms a semidisk-shaped or rectangular surface.

10. 10. A conductive coil according to any one of claims 4 to 9, wherein the inner loop portion is configured to form an involute portion of the coil.

11. 11. A conductive coil according to claim 1, comprising a plurality of pairs of active parts connected in series with each other, adjacent pairs of active parts overlapping in the circumferential direction to define a space of a second type housing a magnetic flux guide, the space of the second type being a circumferential space between two adjacent active parts of different pairs of active parts of the coil.

12. 12. The conductive coil of claim 11, wherein the number of active pairs is an integer multiple of two.

13. 13. A conductive coil as claimed in claim 11 or 12, arranged in use for current to flow in the same direction along adjacent active parts of the coil separated by one of the second type of spaces for guiding magnetic flux.

14. 14. A conductive coil according to any one of claims 11 to 13, wherein the pairs of active parts are integrally formed or formed by serially connecting a number of separate elements, each of which includes a pair of active parts.

15. 15. A conductive coil according to any one of claims 1 to 14, wherein the first and second connection portions extend parallel to an axis of rotation.

16. 16. A conductive coil according to any one of claims 1 to 15, wherein the first and second connection portions of the coil are provided adjacent a radially outer end of the coil.

17. 17. A stator for an axial-flux electric machine comprising a plurality of conductive coils according to any one of claims 1 to 16, said plurality of conductive coils being circumferentially distributed around the stator.

18. 20. An axial-flux electric machine comprising a stator as claimed in claim 17, the machine comprising a pair of opposed rotors disposed on either side of the stator.

19. 1. A method of manufacturing a stator for an axial-flux electric machine having flux guides and distributed windings, comprising the steps of: positioning a plurality of conductive coils within the stator housing so as to be circumferentially distributed about the stator housing, each conductive coil comprising a pair of circumferentially overlapping conductive elements connected in series with one another; each conductive element comprising a pair of active parts including a first active part and a second active part, each active part extending in a radial direction perpendicular to a rotation axis of the electric machine; a plurality of winding turn portions stacked parallel to the axis of rotation such that a cross section perpendicular to a radial direction of each active portion is elongated with a long dimension parallel to the axis of rotation, the second active portion being circumferentially spaced and axially offset from the first active portion; And, connecting said plurality of conductive coils to a connecting means such that said plurality of conductive coils are connectable to a power source.

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